Sensing Human Touch with Low Frequencies

Passive touch sensing flexible keyboards could supplement low power displays.
Sensing Human Touch with Low Frequencies. From touch screens to keypads, the technology that senses a human touch is vital to communication in the 21st century. This technology found in the displays of smartphones and tablets requires active touch sensing technologies. In other words, the display requires power delivered via battery or another power source, which consume precious energy and require users to charge their devices more frequently. On the other hand, passive touch sensing technologies, which don’t require a power source, can be utilized to implement low power or energy harvesting interfaces.
Hatem Elfekey, a graduate student from Nagoya University’s Graduate School of Engineering in Nagoya, Japan, and his team recently designed a keyboard with a touch keypad interface, a signal conditioning circuit, and a controller to test the passive touch sensing technique from AC hum – a phenomenon that uses extremely low frequency electromagnetic fields. Human body tissues exhibit conductivity at very low frequencies, such as 50-60 Hz. This conductivity creates a small signal on the skin of the body, which, when detected by the touch surface, can be utilized to detect a human touching an object like a keyboard. A matrix of dry electrodes, developed mainly from bare copper wires and insulators each about one-millimeter-thick, was created on a standard plastic sheet. The plastic sheet, or the substrate layer, allows the keyboard to be flexible. The team also recorded keyboard measurements to demonstrate the difference between when a finger touches the keyboard and when it doesn’t, known as a touch event and a non-touch event respectively. The results showed that the minimal detectable distance for a touch event to occur was less than 0.5 mm. Distances of 1 mm, 5 mm, 10 mm, and 20 mm from a finger to the keyboard were also tested and showed a voltage signal of less than 1 V at the output, compared to a voltage signal of 5 V when the finger was less than 0.5m from the keyboard.
The touch sensing keyboard presented in this project is a unique solution for conserving power consumption in touch sensing displays. This keyboard, a passive device, doesn’t require an external power source and can work via voltage signals from the human body. Human touch may continue to be an important communication tool with high-level machines, just at a much less power cost to the machine itself.